1. Field of the Invention
The present invention relates to a three-phase autotransformer, and particularly to a three-phase autotransformer with a balancing function which can eliminate imbalance between voltages and currents of the three phases, thereby improving efficiency of electric apparatuses connected to the autotransformer.
2. Description of Related Art
When a conventional three-phase autotransformer is used to supply power to various electric apparatuses connected to its output, imbalance between the three phases may occur.
FIG. 1 is a diagram for illustrating mechanism that causes the imbalance. Three-phase output terminals of a distribution transformer 101 are connected, through distribution lines 102U, 102V and 102W, to the input terminals U, V and W of a three-phase autotransformer 103 including three-phase windings 103U, 103V and 103W which are star-connected. One end of each winding is connected to the neutral point N, which in turn is connected to the neutral point of the distribution transformer 101 through a distribution line 102N. Three-phase output terminals u, v and w are brought out of the windings, and an induction motor M is connected to the output terminals. In addition, an electric heater H is connected between the neutral point N and the output terminal v. The U-phase winding 103U consists of a con, non winding from the neutral point N to the output terminal u, and a series winding from the output terminal u to the input terminal U. Likewise, each of the windings 103V and 103W consists of a common winding from the neutral point to the output terminal, and a series winding from the output terminal to the input terminal.
With this connection, although the induction motor M will keep balance of the three phases, the electric heater H may disturb it, thus causing differences in voltages and currents between the phases. In the case of FIG. 1, for example, the current of the V-phase is greater than the currents of the other phases, which will cause a voltage drop due to a resistance of the distribution line 102V of the V-phase. Thus, imbalance between voltages will occur as well as the imbalance between currents. The imbalance will have various harmful effects on electric apparatuses connected to the transformer. For example, the torque of the induction motor M may be reduced, and its efficiency may be decreased owing to an increase in the slip. In addition, the windings of the induction motor may be overheated, thereby shortening its life.